1. Field of the Invention
This invention relates to a method and apparatus for the continuous on-site purification of ultrapure liquids, especially liquids used in a semiconductor wafer cleaning process, such as ultrapure peroxydisulfuric acid and sulfuric acid solutions.
2. Description of the Prior Art
In the past, it has been common practice in industries requiring chemicals, especially ultrapure chemicals, to utilize such chemicals until a certain degree of contamination was reached. At that point, it was necessary to remove the contaminated chemicals from the process apparatus, clean the apparatus, and add new chemicals as needed. Contaminated chemicals were commonly disposed of by any convenient means. This has included legal and illegal dumping in land areas and occasionally in waterways.
In the semiconductor industry it is important to remove all organic and inorganic particles from the surface of semiconductor wafers. This is commonly done by immersion in an acid bath. A preferred acid bath consists of an oxidant solution of sulfuric acid and either peroxydisulfate ion, which has the formula S.sub.2 O.sub.8 .sup.-2, or hydrogen peroxide and ultrapure water. The oxidant solution is commonly made by mixing together the oxidant and sulfuric acid. This combination produces a highly oxidizing compound which attacks carbon or other organic particles on the surface of the wafers.
The wafers are commonly held in a cassette boat whereby they can be cleaned by immersion into a tank containing the oxidant solution. The time for immersion is usually about ten to twenty minutes. After immersion, the cassette boat containing the wafers is then washed in ultrapure water. The purity of the water is determined by measuring the resistivity of the water.
In prior art processes very high purity sulfuric acid and H.sub.2 O.sub.2 or a source of peroxydisulfate are required. The bath temperature is maintained at about 80.degree. C.-150.degree. C. In about one half hour, contamination of the acid takes place with an increased concentration of particles. At this time, the acid is normally dumped and a new bath of high purity acid is added.
In recent years two developments have made this approach undesirable. The first of these has been the requirement of increasingly greater purity of chemicals, especially in industries such as the semiconductor and pharmaceutical industries. The second development has been an increase in concern for the environmental effects of the dumping of hazardous waste materials in the sewer lines, as well as on land.
With regard to the purity of chemicals, it is evident that the purity of a liquid over a period of time is greater at the start of a process time period than it is at the end of that time period. As greater purity has become more and more important, it has become apparent that higher quality is produced using chemicals during the first part of the period when purity is greater, than at the end of the tolerable processing period when contaminants have been able to build up in the chemical liquid. As a consequence, in the specific case of the cleaning of semiconductor wafers using peroxydisulfuric acid, the wafers cleaned at the beginning of the process period have a higher quality than those which are cleaned at the end of the tolerable contaminant processing period.
With respect to the dumping of hazardous chemicals, public awareness coupled with recently passed hazardous waste chemical disposal laws, have made the disposal of hazardous chemicals extremely difficult as well as costly.
In addition, the necessity of periodic replacement of chemically pure liquids represents an increased cost of materials, increased labor costs, as well as a small but real risk of contamination or hazard to the personnel involved. Finally, there is the cost involved in shutting down a process for whatever time is required to replace the chemicals.
In addition, any time chemicals are stored or transferred, impurities are introduced which are intolerable for ultrapure requirements. For example, stabilizers often must be added to prevent decomposition of unstable compounds. Also, reaction with the containers during storage and transfer, although slight in most cases, often produces a contamination level in such liquids which is intolerable for ultrapure process requirements.
Manufacturing space available for semiconductor manufacturing is often limited and expensive. Moreover, local government codes often limit the amount of sulfuric or other acid which can be inventoried in any one container.
In the case of semiconductor wafer cleaning various chemicals can be used. One process utilizes hydrogen peroxide which must be shipped with stabilizers in order to prevent spontaneous decomposition. The stabilizers which are required to be used introduce impurities which will ultimately contaminate the wafers during the cleaning process.
Another process utilizes potassium or ammonium peroxydisulfate. Potassium peroxydisulfate commonly contains metal ions as impurities which produces a known problem with integrated circuits, particularly MOS circuits.
While ammonium peroxydisulfate could theoretically be made quite pure, such purity levels are not available on an economically attractive basis.
In light of the above difficulties in requirements for the use of ultrapure chemicals and the subsequent contamination and disposal requirements, it is desirable to provide a method and apparatus capable of maintaining purity of the ultrapure liquid throughout the course of the reaction which will avoid contamination buildup. In addition, it is desirable to provide a process and apparatus which avoid the need for the disposal of large amounts of hazardous chemicals. Finally, it is desirable to provide a process and apparatus which reduce processing costs by reducing the amount of chemicals required, reducing the number of personnel involved, increasing the safety of the personnel involved, and eliminating the frequent requirements for shutdown of the process for purposes of renewing ultrapure liquids.
One object of the invention is to provide a novel method and apparatus for distillation of liquids to remove soluble impurities and insoluble particles, especially particles having a diameter of less than 10 microns.
Prior art glass distillation columns are commonly spherical in shape, having a diameter of about two feet and a capacity of approximately 30 gallons. The output is about 600 ml per minute of sulfuric acid. The liquid boiling volume in the boiling pot is about 15 gallons providing a liquid/vapor boiling surface of about 3 square feet at the boiling surface and a heat transfer area of about 6.5 square feet.
The distillation column of the invention is preferably a cylinder of about 6 feet in overall height including a cylindrical bottom having a diameter of about 8 inches and a boiling liquid height of about 2 feet for the same throughput of 600 ml per minute. The boiling volume is about 5 gallons which is one-third of the prior art distillation column. However, the heat transfer area is about 5 square feet with a boiling surface area of about 1/3 square foot. This design saves about 1 foot in furnace diameter at the base when the heating element is included and reduces the inventory of boiling acid by one-third. This is a particularly desirable safety feature. Other features of this invention allow for improved system performance even with a smaller heat transfer area.
Nucleation sites are required to avoid bumping which can be very dangerous when acid is distilled since the acid can be forced out of the distillation column. Metal boilers are inherently rough which provides nucleation sites for boiling. However, metal boilers are unsuitable for the invention process since they introduce metallic contaminants. Most distillation in glass boilers generally utilize boiling chips in the form of rough ceramic inert granules to provide nucleation sites since the glass surface is extremely smooth and does not inherently provide nucleation sites. It was found that use of these chips was unsatisfactory as the chips scoured the glass over time and created particles which contaminated the sulfuric acid.
This problem is overcome by the distillation column of the invention. A particularly novel feature of the distillation column is the inclusion of porous fused quartz or other glass boiling rings which are spaced from and fused at selected points to the distillation column walls.
The preferred material for these boiling rings is composed of Quartz Scientific "TPL".TM. in the form of fused quartz which is formed by fusing silicon dioxide powder with heating from one side. This results in an inner surface which is smooth and glassy and an outer surface which is highly textured. The textured surface provides interstices for vapor nucleation. The resulting piece is sawed into rings and mounted in the distillation column spaced from the wall.
It was found that a particularly novel advantage of locating boiling rings spaced from the distillation column wall is that a novel smooth convective boiling pattern is created. Boiling with bubble and vapor formation takes place in the highest heat zone which is nearest the column walls. This is believed to cause a predominant circulation pattern wherein the vapor rises smoothly up the interior wall surfaces of the distillation column on both surfaces of the boiling rings. The resultant smooth convective flow reduces entrainment of contaminants, particularly soluble and insoluble particles of less than about 10 microns in size.
At the same time with the distillation column of the invention, cooler unvaporized liquid also rises to the surface but returns to the bottom of the column in the relatively cooler center portion. Not only does this feature insure smooth, energy efficient boiling but there is also a reduction in the boiler wall temperatures.
Wall temperatures are a direct function of the efficiency of heat transfer from the heating zone or furnace to the liquid since boiling cools the walls by absorbing the energy. The distillation column of the invention provides a higher efficiency of heat transfer than does the prior art method by causing boiling to take place at the wall due to the spaced boiling rings. This advantage not only reduces the required heat transfer area and energy costs but also provides a smooth convective boiling pattern which reduces entrainment of particles.
One of the most advantageous features of the distillation column of the invention is the ability to remove both soluble impurities and insoluble solid particles of less than about 10 microns.
In order to produce ultrapure liquids, it is particularly necessary to remove particles, particularly insoluble small particles in the range of one micron and smaller sized particles. When distillation is used as a method for purification, there are problems associated with the removal of small particles. This is due to the fact that the small particle, for example a particle of less than 10 microns in size, can be expected to be carried over by the vapor during distillation since such particles would have a mass/cross sectional area that would prevent gravity separation from the vapor flow.
Not only are the particles expected to be released into the vapor stream during the agitation caused by boiling, but also it would be expected that as the bubble forms the particle or particles would be carried to the surface on the bubble liquid interface and ejected into the vapor during the distillation. Thus, it would be expected that all small particles would be entrained in the rising vapor stream to be carried over to the product reservoir.
It is an object of the invention to provide a distillation apparatus and method which clearly separates small particles, especially particles of less than 10 microns, from the vapor distillate in contrast to the expected entrainment thereof.
A combination of features of the distillation column of the invention have made possible the effective removal of insoluble impurities and non-volatile solid particles having a size down to the limits of liquid particle counters, i.e. 0.2 microns.
One novel feature of the distillation column of the invention which is believed to contribute to particle removal includes in particular the use and location of the boiling rings which optimize smooth boiling at the liquid/vapor interface and provide efficient heat transfer resulting in fewer particles being expelled into the vapor stream. The resulting smooth convective flow of vapor bubbles upwardly along the walls and the downward flow of cooler liquid substantially centrally of the distillation column is also believed to be novel.
Another feature of the distillation column of the invention which is believed to contribute to particle removal is the provision of a packed column as an efficient counter current particle scrubber which enables the reflux stream to continually wash the rising vapor and particles back down the column.
Another feature of the distillation column of the invention is the provision of redirector rings which together with the packing cause comingling of the acid and vapor to effect further scrubbing of the vapor/reflux liquid streams. The redirector rings also direct condensed vapor to fall within the central area of the distillation column.
Still another novel feature of the distillation column of the invention which is believed to contribute to particle removal is the provision of a low net vapor velocity by sizing the distillation column with a relatively large diameter in relation to the throughput. This permits increased dwell time for rising vapor to be scrubbed by counter current downward flow of condensed liquid.
The use of glass in apparatus for distillation gives rise to problems which are unique to such use. These include among others, the brittleness of glass, and in the case of distillation of highly corrosive liquids the consequent risks involved if glass is broken. Moreover, glass is not flexible so special care and design are needed to provide joints which will flex and not break or leak upon expansion of glass during the relatively high distillation temperatures, for example 300.degree. C.
The distillation apparatus of the invention provides a design which overcomes the above problems unique to glass and at the same time is of a relatively small size to limit the quantity of corrosive liquid which must be handled. This reduces the risk inherent in the event of breakage and at the same time operates within existing city code limitations with respect to the total amount of corrosive material which can be inventoried.